This invention relates to a method and apparatus for stabilizing an aluminum metal layer in an aluminum electrolytic cell. Electrolysis of aluminum is usually carried out by serially connecting a plurality of rectangular electrolytic cells through anode and cathode bus bars to form a pot line or cell group and passing a large DC current of the order of 50 to 300 kiloamperes through the pot line to electrolyze alumina contained in respective cells. A well known arrangement of the electrolytic cells is of a so-called double entry type in which the electrolytic cells are arranged in a side-by-side relation or an end-to-end relation with respect to the direction of flow of the current, so as to supply current from both sides of each cell. With this type of the cell arrangement, since the cathode bus bars carrying large current extend along the side surfaces of the electrolytic cells a strong magnetic field is created in the electrolytic cells.
In each cell, the current supplied from an anode bus bar flows to an electrolytic bath through one or more anode electrodes to reach an aluminum metal layer formed as the result of electrolysis, then flows to a cathode bed carbon to be collected by a plurality of cathode bars disposed parallel with the shorter end wall of a steel container and finally is taken out through a cathode bus bar extending along the longer side wall of the steel container. While being collected by the cathode bars, the cell current tends to concentrate in a current path having a small electric resistance so that a portion of the current flown out from an anode electrode at the central portion of the cell does not flow through a path immediately below that anode and perpendicular thereto but instead flows directly through a path leading to a cathode bar disposed near the longer side wall of the steel container. As a consequence, the current flows in the horizontal direction in the cell, particularly in an aluminum metal layer, from the longitudinal center line of the cell to the longer side wall of the steel container. Such horizontal current also flows through the aluminum metal layer when a solidified bath or freeze formed on the cell wall or sludge in the aluminum metal electrically insulates the cathode bed carbon during the operation of the cell.
The horizontal current in the aluminum metal layer undergoes natural action with the magnetic field to agitate or fluctuate to form curved or oscillatory surfaces on the aluminum metal layer. Especially, when the vertical component of the magnetic field has its inclination to the horizontal direction it produces a nonuniform pressure distribution in the aluminum metal which enhances the curved state on the upper surface of the aluminum metal layer.
When the aluminum metal layer becomes unstable as above described, the aluminum metal layer may come into direct contact with the lower surface of the carbon anode electrode with the result that the current flows through such contacted portion, thereby greatly decreasing the current efficiency.
As a result of an exhaustive investigation, I have found that the aluminum metal layer can be efficiently stabilized where a ferromagnetic member is horizontally disposed above or below the aluminum metal layer so as to cause the vertical component of the magnetic field created by the ferromagnetic member to cancel the vertical component of the magnetic field created by the cell itself thereby decreasing the inclination or gradient of the vertical component.